Reflective optics produce no chromatic aberrations and are thus usable over many wavelengths without degradation. Their reflective nature results in light being bent back in the direction in which it originally came. This often limits the field of view and f/number because mirror aperture start to interfere with optical rays, resulting in vignetting. However, this same attribute results in a reduced overall length of the optical system because the optical path is folded upon itself. The more reflections that are used, then the shorter the overall length can be while maintaining the same focal length. The decreased length however will result in a great obscuration of the aperture for an on-axis optical system, resulting in a reflective annular aperture optical system.
Reflective annular aperture optics have been designed and demonstrated as reflective imager components. (See, Tremblay, E., Stack, R., Morrison, R., Ford, J., “Annular folded optic imager,” Proceeding SPIE 6232, 2006.) It is desired to use these types of optics as multi-FOV optical systems. The challenge lies in creating an all reflective optical system that maintains the compact nature. Some work on reflective zoom system has been performed, but these concepts are difficult to apply to the compact reflective imager. (See, Chang, J., Wang, Y., Zhang, T., Talh, M., Weng, Z., Yang, H., “All reflective zoom systems for infrared optics,” Optical Society of America IODC Proceedings, 2006.) Another method of creating a multiple field of view system is via the insertion and removal of an afocal of a given magnification power that will change the focal length of the optical system while maintaining the same image plane. This is a common method in traditional optical systems. For a compact optical system the difficulty lies in the creation of a compact afocal to be added to the compact imager optic. (See, also, Tremblay, E., Stack, R., Morrison, R., Ford, J., “Thin cameras using annular folded optics,” Applied optics volume 46, No 4, Optical Society of America, 2007.)